This invention relates to an apparatus for preventing leakage of a portion of a coarse powder into a fine powder discharge duct of a classifier.
It is well known for a classifier to have a rotary classifying impeller in a machine housing or casing in which an introduced raw material is classified into a coarse portion being discharged below and a fine portion, being made to flow through the classifying impeller together with an air flow.
However, in such prior classifiers, there can be gaps at the boundary portions between the rotating classifying impeller and a fixed end of the fine powder discharge duct, and thus the coarse particles can leak into the fine powder discharge duct through such gaps.
Therefore, in order to prevent leakage of the coarse powder through the gaps at the boundary portion between the classifying impeller and the fine powder discharge duct, in the prior art a labyrinth mechanism and a mechanical seal were provided at those gaps, or means for blowing seal gas by is provided. However, none of such attempts can perfectly prevent the leakage.
Although the pressure difference between the outside and the inside of the labyrinth is reduced as a result of the repeated expansion and compression through the labyrinth, this is not a perfect seal structure, because there still remains such problems as powder piling up in the labyrinth. As for mechanical seals, contact sliding portions between the rotary side and the stationary side are easily worn, due to powder abrasion. In addition, the complicated mechanism thereof consequently increases the production costs. With sealing gas, the leakage of coarse particles from the classifier will be perfectly prevented. However, an excess or a shortage of the blowing volume of the seal gas will easily influence performance and running costs. In addition, this measure requires a supplemental air-supply installation, increasing initial investiment.